Access systems or apparatuses such as wheelchair lifts assist mobility-challenged individuals. For example, parallelogram-type lifts may be used with vehicles in personal and public (such as, paratransit) mobility applications. Such parallelogram-type lifts may use a hydraulic system to move a transfer member, which may include a lift, between three positions including stowed (which may included folding), transfer level (such as the floor level of a vehicle or the level of other surfaces), and ground level positions. Such hydraulic systems may take advantage of gravity to lower the lift transfer member from the transfer level to the ground level. This “gravity down” or “gravity assist” feature may conserve power and reduce the wear on the components of the hydraulic system, such as the hydraulic pump and motor. The gravity down feature, which may include unfolding and lowering operations, may be controlled by throttling the hydraulic fluid flow by using, for example, flow restrictors, actuatable valves, and the like to limit free-fall of the lift and provide smooth motion of the lift transfer member, particularly when transferring a user between the transfer and ground positions. In such hydraulic systems, the motor and hydraulic pump may operate to raise the transfer member from the ground level to the transfer level, and to move the transfer member into a stowed position.
Access systems or apparatuses may include safety systems to ensure the well being of access apparatus users. The National Highway Transportation Safety Administration (NHTSA) has adopted rules mandating the implementation of safety systems, such as “interlocks.” One type of interlock prevents the access apparatus from being stowed when the transfer member is occupied. To detect whether the transfer member is occupied, a safety system may include mechanical, electrical, or electromechanical sensors. An example of such a sensor includes a hydraulic pressure switch, which may be set or calibrated to detect pressures over a predetermined threshold that are indicative of a load on the transfer member. For example, the threshold may equal about 50 pounds. When the hydraulic pressure switch detects a pressure about equal to or greater than the threshold, the switch changes states to disconnect the pump motor from a power source.
The use of a hydraulic pressure switch has disadvantages. In some circumstances, such as when the motor is energized to start stowing the transfer member, the pump generally needs to run for a time period to pressurize the hydraulic system before the pressure switch can detect whether the transfer member is occupied. During this time period, however, the pump may build up sufficient pressure in the system to initiate stowing of the transfer member, even if the transfer member is occupied. Further, the access system may continue to stow the transfer member after the pump has been shut off until the hydraulic system reaches a steady state. Steady state may be reached, for example, when the pressure in the system becomes balanced with that of a load supported by the transfer member. Additionally, if the system pressure drops below the threshold when the pump is deactivated, the hydraulic pressure switch may again turn on the pump causing erratic or pulsating operation of the access system. Such operation may lead to pressure spikes, which may damage the components of the hydraulic system. Moreover, such operation may be disconcerting to an occupant of the access system, and may cause the occupant to fall from the access system.